Method for compensating for defective printing nozzles in an inkjet printing machine

Abstract

A method for compensating for defective printing nozzles in an inkjet printing machine by way of a computer. Defective printing nozzles are compensated for by an increased ink drop volume of neighboring printing nozzles and the real positions of the print dots of all printing nozzles are determined and, to compensate for a printing nozzle failure, the required ink drop volumes of the respective neighboring printing nozzles are calculated for every printing nozzle as a function of the real positions of the print dots of the respective neighboring printing nozzles. In addition to increased ink drop volumes of the respective neighboring printing nozzles, reduced ink drop volumes are calculated for the respective next but one printing nozzles and all ink drop volumes are calculated based on the print dots of the respective printing nozzles.

Claims

1. A method of compensating for defective printing nozzles in an inkjet printing machine by way of a computer, the method comprising: to compensate for defective printing nozzles, increasing an ink drop volume of neighboring printing nozzles; determining real positions of print dots of all printing nozzles; to compensate for a printing nozzle failure, calculating required ink drop volumes of respective neighboring printing nozzles for each printing nozzle as a function of the real positions of the print dots of the respective neighboring printing nozzles; in addition to increased ink drop volumes of the respective neighboring printing nozzles, calculating reduced ink drop volumes for respective next but one printing nozzles; and calculating all ink drop volumes as a function of the print dots of the respective printing nozzles; calculating the increased and reduced ink drop volumes of the respective next and next but one printing nozzles, respectively, with the computer by initially calculating values of the increased and reduced ink drop volumes on a basis of optimum print dots of the next and next but one printing nozzles, respectively, and subsequently determining dependencies of the increased and reduced ink drop volumes on the real print dots of the next and next but one printing nozzles, respectively, on a basis of calculated values, and subsequently calculating real values of the increased and reduced ink drop volumes by applying the determined dependencies.

2. The method according to claim 1, which comprises determining the dependencies of the increased and reduced ink drop volumes on the real print dots by taking measurements on inkjet test stands or by simulation.

3. The method according to claim 1, which comprises calculating with the computer the increased and reduced ink drop volumes of the respective next and next but one printing nozzles additionally or alternatively by determining and applying the dependencies of the increased and reduced ink drop volumes on the amplitude of the respective printing nozzles.

4. The method according to claim 1, which comprises describing the dependencies of the increased and reduced ink drop volumes on the real print dots of the next and next but one printing nozzles, respectively, in characteristic curves and calculating the dependencies during the application by the computer by interpolation.

5. The method according to claim 1, which comprises taking into consideration interdependencies between the real print dots of the next and next but one printing nozzles, respectively, for calculating the increased and reduced ink drop volumes of the respective next and next but one printing nozzles, respectively.

6. The method according to claim 1, which comprises calculating with the computer the increased and reduced ink drop volumes of the respective next and next but one printing nozzles, respectively, also for and based on the print dots and/or the amplitude of the neighboring printing nozzles that are farther away.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a schematic illustration of a system of a sheet-fed inkjet printing machine.

(2) FIG. 2 illustrates a sample sheet exhibiting a stylized defect caused by a missing nozzle.

(3) FIG. 3 illustrates an example of a printing machine system including an image recording system.

(4) FIG. 4 illustrates an example of the position and calculation of the ink drop volumes.

(5) FIG. 5 is a flow chart of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, the field of application of the preferred exemplary embodiment is an inkjet printing machine 7. FIG. 1 shows an example of the fundamental design of such a machine 7, including a feeder 1 for feeding the printing substrate 2 to the printing unit 4, where it receives an image printed by print heads 5, and a delivery 3. The machine is a sheet-fed inkjet printing machine 7 controlled by a control unit 6. While this printing machine 7 is in operation, individual printing nozzles in the print heads 5 in the printing unit 4 may fail as described above. Such failures result in white lines 9 or, in the case of multicolor printing, in distorted color values. An example of such a white line 9 in a printed image 8 is shown in FIG. 2.

(7) An example of a printing machine system 15 that may be used is shown in FIG. 3. In addition to the printing machine 7 itself, the system 15 consists of an image recording system 17, which comprises at least one image sensor, at least one camera 16 that is integrated into the printing machine 7. The at least one camera 16 records the images 8 generated by the printing machine 7 and transmits the data to a computer 6, 19 for analysis. This computer 6, 19 may be a separate computer, e.g. one or more dedicated image processors 19, or it may be identical with the control unit 6 of the printing machine 7. In addition, the entire process is controlled by a workflow system 18. The latter provides assistance to an operator 20; depending on the situation, this may for instance be the person processing the print job or the operator at the machine 7 itself.

(8) A preferred embodiment of the sequence of steps of the method of the invention is schematically shown in FIG. 5. In accordance with the method proposed herein, the computer 6, 17 selects the compensation strength 13 and the dilution 14 of the printing nozzles that contribute to the compensation as a function of the printing positions/print dots and/or amplitudes thereof.

(9) The first step is to measure the jetting properties of the printing nozzles such as the printing positions, amplitudes, and degrees of variance of the printing nozzles in the course of a defective nozzle detection process. This is done by printing and analyzing control elements such as specific printing nozzle test charts 21. These test charts 21 are then recorded and digitized by the image recording system 17. On the basis of the printed and recorded printing nozzle test charts 22, a decision is made whether defective nozzles 11 are to be switched off; in addition, all the data required for an implementation of the described compensation method are now known.

(10) In a second step, the default compensation strengths, which can also be considered as compensation factors KF.sub.std and dilution factors DF.sub.std are determined. As explained above, the immediate neighboring nozzles 13 are amplified (compensation strength) whereas the next but one neighboring nozzles 14 are diluted (dilution). This is shown in FIG. 4 in the form of a schematic overview 10. In the illustrated example, a printing nozzle 4 is switched off because it jets at an angle, i.e. its print dot deviates to a considerable extent. This defective printing nozzle 11 is compensated for by printing nozzles 3 and 5, with KF>1 while nozzles 2, 6 are diluted with DF<1. Printing nozzle 5 likewise jets at an angle, i.e. has a deviated print dot 12, but may still be used in the printing operation and thus remains activated.

(11) To determine KF.sub.std and DF.sub.std, specific printing nozzle test charts 21 are printed with nozzles that are artificially switched off while area coverage, compensation factor (KF3=KF5) and dilution factor (DF2=DF6) of the neighboring printing nozzles 13, 14 are varied. The optimum range/optimum values of these parameters are determined by visually examining the image 8 to see whether the defective printing nozzles 11 that have been compensated for in the printed image 8 can still be visually recognized. The values that are determined in this way on average correspond to the average, correct print dots 23.

(12) The next step is to determine the sensitivities 24, i.e. dependencies, a.sub.xy of the factors KF.sub.3 and FK.sub.5 as well as DF.sub.2 and DF.sub.6 on the print dots 23 of the printing nozzles X.sub.2 to X.sub.6 for compensation 13 that is visually no longer discernible. This may be done on an inkjet test stand or by simulation. In the example shown in FIG. 4, in which compensation 13 for printing nozzle 4 is implemented, the sensitivities are as follows:

(13) TABLE-US-00001 a.sub.22 = DF.sub.2/ a.sub.23 = DF.sub.2/X.sub.3 a.sub.25 = DF.sub.2/X.sub.5 a.sub.26 = DF.sub.2/X.sub.6 X.sub.2 a.sub.32 = KF.sub.3/ a.sub.33 = KF.sub.3/X.sub.3 a.sub.34 = KF.sub.3/X.sub.4 a.sub.36 = KF.sub.3/X.sub.6 X.sub.2 a.sub.52 = KF.sub.5/ a.sub.53 = KF.sub.5/X.sub.3 a.sub.54 = KF.sub.5/X.sub.4 a.sub.56 = KF.sub.5/X.sub.6 X.sub.2 a.sub.62 = DF.sub.6/ a.sub.63 = DF.sub.6/X.sub.3 a.sub.65 = DF.sub.6/X.sub.5 a.sub.66 = DF.sub.6/X.sub.6 X.sub.2

(14) Subsequently, the last step of the method is to compensate for the defective printing nozzles 11. Once printing nozzle 4 has been switched off, the printing nozzle positions or rather the deviations thereof from the target positions of the print dots that are known from the measurement of the printing positions of printing nozzles 4 to 6 may be used to calculate compensation factors KF.sub.3 and KF.sub.5 and dilution factors DF.sub.2 and DF.sub.6, which may then be applied. Depending on the printing positions of the contributing printing nozzles, these factors no longer correspond to the standard factors.
DF.sub.2=DF.sub.std+a.sub.22*X.sub.2+a.sub.23*X.sub.3+a.sub.25*X.sub.5+a.sub.26*X.sub.6
KF.sub.2=KF.sub.std+a.sub.32*X.sub.3+a.sub.33*X.sub.3+a.sub.35*X.sub.5+a.sub.36*X.sub.6
. . . etc.

(15) The advantages of the method of the invention are that all defective printing nozzles 11 that have been switched off are correctly compensated for, no white lines 9 remain, and no dark lines are created. This more than compensates for potential disadvantages such as an increased computational effort including and increased memory space requirements to determine the dependencies.

(16) In a further preferred embodiment, it is analogously possible to determine the dependencies KF.sub.3, 5 and DF.sub.2, 6 on the amplitudes A.sub.x of printing nozzles 2 to 6 and the combinations thereof and to take these dependencies into consideration when the defective printing nozzles 11 are compensated for.

(17) Another approach to a solution is an iterative application of the sigma-delta process, which is carried out analogously to the density compensation, taking into consideration the print dots and/or amplitudes and/or variances.

(18) Further preferred embodiments comprise: The method of the invention may be simplified by making certain assumptions, for instance by assuming symmetry or DF.sub.2=u*KF.sub.3, u being a linear factor. The method of the invention may additionally be extended to include further neighboring nozzles such as nozzles 1 and 7 in the example shown in FIG. 4 and even further neighboring printing nozzle pairs. If the sensitivities 14 are not constant but depend on the print dot deviations, they may be described by characteristic curves and calculated by interpolation in the application. The method that has been described so far does not factor in interdependencies between the deviations. If these are present, they need to be determined using more complex methods such as DoE. Determination of the position-dependent factors by the formation of a model. Printing nozzle 11, which jets at an angle and is factually defective. is not switched off and compensation factors KF.sub.x for the printing nozzles 13, 14 neighboring the white line 9 and dilution factors are determined analogously with the method of the invention. In this case, the influence of the printing nozzle 11 that jets at an angle and is factually defective of course needs to be taken into consideration.

(19) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 1 feeder 2 printing substrate 3 delivery 4 inkjet printing unit 5 inkjet print head 6 computer 7 inkjet printing machine 8 entire print 9 white line 10 schematic overview 11 defective printing nozzle that has been switched off 12 active printing nozzle that prints incorrectly 13 neighboring printing nozzles jetting an increased volume of ink (compensation) 14 neighboring printing nozzles jetting a reduced volume of ink (dilution) 15 printing machine system 16 camera 17 image recording system 18 workflow system 19 image processor 20 operator 21 printing nozzle test chart 22 printed and recorded test chart 23 correct print dots 24 determined sensitivities